This invention relates to an air filter material for use in conjunction with, for example, automobile engines and air-conditioning systems.
Conventionally known filter materials which have been generally used in the above-noted fields are (1) filter papers made of rayon, cotton, or wood pulp as raw materials, (2) filter papers of the group (1) impregnated with oils, (3) simple laminates of two fibrous layers of different bulk densities bonded to each other with an adhesive, each layer consisting of synthetic fibers, as disclosed in U.S. Pat. No. 3,400,520 (hereinafter this type of filter materials is referred to as "conventional density-gradient type nonwoven synthetic fiber fabric"), (4) synthetic resin foams, and (5) the synthetic resinfoams of the group (4) impregnated with oils.
However, being a surface filtration type, the materials of the group (1) must have a large filter area and, hence, have a disadvantage of requiring increased man-hours in making a filter element for use in automobiles; because of the oil-impregnated type, the materials of the group (2l ) must be made of a filter paper having a large pore size and, in addition, a large porosity in order to prevent the increase in restriction to airflow caused by the impregnant oil, resulting in decreased air-filter efficiency. The filter material of the group (3), which is a laminate of two nonwoven fabric layers of different bulk densitites made of customary synthetic fibers (polyester fiber, polyvinyl alcohol fiber, and rayon), is still insufficient in dust holding capacity. The materials (4) and (5) are also unsatisfactory because of the widely dispersed pore size and porosity and, in addition, inferior air filter efficiency.